Propane, which is a naturally occurring material and by-product of many petroleum refining processes, has a low economic value and is often burned as a fuel gas or used as a component in liquefied petroleum gas (LPG). Efforts have been made to upgrade propane to move valuable hydrocarbons such as butanes, higher aliphatics, iso-alkanes and aromatics. The isobutane product is a valuable material for the alkylation of lower molecular weight olefins to product gasoline boiling range hydrocarbons. The aromatics product and normal butane can be blended into gasoline to enhance octane rating or can be employed as a petrochemical feedstock.
The prior art suggests a number of methods and process conditions for upgrading a propane feedstock. Many of these methods provide for a mixture of propane and an olefin as feedstock for a catalytic conversion process.
U.S. Pat. No. 3,775,501 (Kaeding, et al.) discloses a process for improving the yield of aromatics from a hydrocarbon such as propane (claim 21) with a crystalline aluminosilicate zeolite catalyst. There is no evidence that a mixed feed of propane and an olefin is employed. The reaction is conducted at temperatures of about 260.degree. C. to 700.degree. C.
U.S. Pat. No. 3,845,150 (Yan, et al.) discloses a process for preparing aromatic hydrocarbons over a ZSM-5 type catalyst from a mixed feed containing about 20%-65% by weight saturates and about 20%-50% by weight olefins. The saturates may comprise a C.sub.3 hydrocarbon, and the olefins may comprise propylene. The mixed feed allows for the process to be conducted in a substantially heat balanced condition. There is no disclosure as to the thickness reactivity of the saturated hydrocarbon portion of the feed because of the presence of added olefin. Also, the liquid product resulting from the process comprises substantially insignificant amounts of butanes, and almost all aromatics (e.g., 90% aromatics in Example II).
U.S. Pat. No. 3,960,978 (Givens, et al.) discloses a process for the preparation of a liquid hydrocarbon product comprising predominantly higher olefins and no more than about 20% by weight aromatics, the process employing a mixed feed of C.sub.1 -C.sub.5 paraffins with C.sub.2 -C.sub.5 olefins and a ZSM-5 type catalyst having an alpha value of from about 0.1 to about 120. The yield of butanes from the process is negligible.